Authors:

• Peng Liu, Ph.D., Senior Research Engineer II

• Nafaa Mekhilef, Ph.D., R&D Manager

• Melanie Kuhn, Ph.D., R&D Manager

• Gene Gargas, Technical Applications Manager

• Frederic Laniel, Value Development Director

• Mark Scoular, BSME, Business Development Manager - Aviation

Since the early 1980s, Omniseal Solutions™, a global engineering leader relentlessly dedicated to the design and manufacture of precision sealing and material solutions, has focused on developing new and advanced polymer materials based on polyimide chemistry and formulation for high temperature applications. With the integration of Hycomp, LLC composite materials in the beginning of 2019, Omniseal Solutions™ has now added a new dimension to its solutions portfolio by offering polyimide-based materials with short and long fiber reinforcement for critical applications involving high thermo-mechanical requirements. This integration complements the business' existing polyimide material offerings, primarily its Meldin^® 7000 thermoset solutions that typically excel in demanding high temperature tribological applications where low wear and low friction performances are required.

With trends moving toward improving the efficiency and performance of industrial and mechanical systems, the need for high performance material solutions has become even more stringent. In today’s aviation industry, for example, various types of metals and composite materials are used to manufacture aircraft, with an increasing demand for composite parts. Aviation manufacturers continue to seek even lighter weight material solutions in order to increase engine efficiency as well as reduce operational costs and emissions. These materials are often selected for a variety of applications and systems on an aircraft and can range from large structural components to smaller mechanical components.

This technical white paper will serve as a guide to design engineers and technical experts who are searching for high temperature polymer and composite material solutions in critical industries such as aviation as well as those who are looking to select the most appropriate grades of Omniseal Solutions™ polyimide material solutions.

Omniseal Solutions' polyimide solution made of advanced composite materials can be tailored to specific applications based on the customer’s needs. These materials provide solutions to address industrial challenges that demand aviation-grade polyimide composites for performance bearings and seals (Figures 1 and 2). Based on a fiber reinforced polyimide matrix, Meldin^® 7500 and H310^® materials present excellent structural integrity to sustain high loads and wear at elevated temperatures.

Table 1 shows general information about Meldin^® 7500 and H310^® materials. When compared to the polycondensation-type Meldin^® 7000 series polyimide, the fabrication process of Meldin^® 7500 and H310^® materials enables the inclusion of short and long fibers; thus, optimizing the reinforcement effect of the fillers. Within the business' polyimide material families, Hycomp^TM and Meldin^® materials are complementary in terms of mechanical and tribological performance.

Table 1: General information about Meldin® 7500, 7000 and H310^® materials

The contact pressure and sliding velocity are critical parameters that determine the suitability of materials for part design in applications that involve relative motion under load. The highest value of the combination of pressure and velocity is commonly referred to as the PV limit for a certain material used in a frictional application. This value represents the upper limit of a product in terms of load and speed in which a material can operate properly. However, it is worth noting that a single PV value or limit alone does not adequately characterize the frictional behavior of a material. Other factors such as frictional heat, creep resistance, and wear mechanisms at the mating surface also play significant roles in determining the suitability of a material for a specific frictional application. Therefore, an examination of the intrinsic properties of the material, coupled with mechanical, thermal, wear and other behaviors, can lead to useful information for designing complex parts for critical applications.

Tribology testing methods such as a thrust washer test (TWT) are commonly used to measure the coefficient of friction and rate of wear in frictional contacts. In this study, several combinations of contact pressures and rotational speeds were selected to examine the frictional behavior of Meldin® 7500 and H310^® in an unlubricated wear test configuration that complies with ASTM D3702. In all cases, a five-hour break-in period was utilized to achieve a full round contact before proceeding to the eight-hour test for measuring the coefficient of friction and wear rate. As seen in Figure 3, the dots and open circles in the PV chart denote the PV conditions under which the test was completed without generating excess noise or vibration. The trend lines are derived from the experimental data represented by these dots and open circles and are meant to define the operating window for these materials in frictional contact. Due to the presence of fiber, the frictional heat becomes prominent under certain combinations of pressure and velocity. The elevated temperature at the contact surface, in turn, contributes to an increased wear and friction. 

Figure 3. PV chart showing the combinations of pressure and velocity that were used for testing H310^® and Meldin^® materials. The color map schematically shows the temperature at the contact surface.

Figure 4 shows the performance of Meldin^® 7500, H310^® and Meldin^® 7021 materials under two PV combinations. For high load and low velocity, Meldin^® 7500 and H310^® composites present a lower coefficient of friction (COF) when compared to Meldin^® 7021 material although the polycondensation polyimide material shows a lower COF at moderate load and increased velocity.

A similar trend is observed in below Figure 5, which shows the wear factor of the three materials. When compared to Meldin^® 7021 material, it is observed that Meldin^® 7500 and H310^® composites present advantages in applications that involve heavy loads.

As shown in Table 2 below, the wear and friction properties of H310^® and Meldin^® 7500 composites remain insensitive to the increase in contact pressure. The frictional heat stays moderately low in high pressure and low velocity conditions. Due to the presence of graphite, the Meldin^® 7500 material can operate at higher velocity when compared to the H310^® material. 

In the design of a mechanical system, the material selection is as imperative as the design itself. The wear volume V_lost, wear factor K, load F and sliding distance L are related by equation 1:

Vlost = KxFxL
(Eq. 1)

The failure point is usually manifested by an abrupt increase in the wear rate of the bearing material. One consideration to note is that at P = 40 psi (0.28 MPa) / V = 300 fpm (1.52 m/s) and P = 60 psi (0.42 MPa) / V = 200 fpm (1.02 m/s), the temperature at the contact surface appears to be higher when compared to other conditions. The elevated surface temperature is also accompanied by increased wear rate. The observation suggests that the frictional heat under certain PV combinations can result in a premature failure of the material.

Table 2. Typical values of coefficient of friction (COF) and wear factor (K) at room temperature from unlubricated thrust washer test (TWT) for Meldin^® 7500 and H310^® composite materials. 

*Measurements were conducted at room temperature. The contact area with the metal counter face is 1.274 cm2. A 5-hour break-in time was applied prior to the 8-hour rotational sliding wear test. The average values obtained during the 8-hour tests are reported in Table 2.

Omniseal Solutions' polyimide materials have been validated and specified by several Tier 1 aviation system OEMs and as a result, are being successfully used in critical systems and components on commercial and general aircraft. Design engineers expect a reliable performance in actuation/flight control systems, engine accessories, control and power generation systems as well as landing systems when working with these composite materials. 

Typically, Omniseal Solutions' materials have been used for wear components in linear or rotary motion; however, temperature capability and higher mechanical strength compared to other polymeric materials have made them an advantageous and preferred choice for metal replacement in non-wear applications as well.  

The below lists several critical and successful Omniseal Solutions' composite material applications for consideration in aircraft systems. 

Improving the efficiency of industrial and mechanical systems is driving the need for high performance material solutions. As reflected in the next-generation aviation programs, new system designs pose new challenges in terms of operating conditions for polymer-based parts. Omniseal Solutions' Meldin^® 7500 and H310^® polyimide composites deliver versatile solutions for producing load-supporting parts that can be tailored to specific critical applications. The mechanism of material removal in frictional contact is a complex subject where the results depend on the inherent property of the wear material and application condition.

Depending on the stress distribution and magnitude, ductile deformation (e.g., yielding) and brittle damage (e.g., fracture and cracking) are the two major modes of failure that can occur under frictional contact. This technical white paper discusses the wear performance of Meldin^® 7500 and H310^® at various combinations of pressure and velocity. Results from various tribological tests suggest the following:

1. The combination of pressure and velocity as one PV value does not reflect the full characteristics of a material under sliding wear conditions. In fact, for a material operating upward to a known PV limit, the same material may show different wear behavior for different combinations of pressure and velocity yielding the same PV limit value. 

2. The effect of pressure and velocity should be examined individually since the wear behavior of some materials can be affected more significantly by high-velocity/low-load conditions and vice versa.

3. Material properties such as yield stress and fracture strength are also critical when it comes to material selection and design for bushings and seals. 

4. The net effect of frictional heat and the environmental temperature can result in substantial temperature elevation at the contact surface, thus affecting the failure mode of the material. 

It is worth noting that the data presented in this article only show the performance over a time span of 13 hours including the break-in time. As a result, the data cannot be used to perform lifetime performance calculations but rather used as a design guide for material selection. The ultimate performance of a material will still need to be determined in real life test programs.

Meldin^® 7500 and H310^® composites present combinations of low COF and excellent mechanical strength in unlubricated frictional conditions for low and medium speeds. H310^® material has proven to be a trustworthy solution for producing bushings and guide rings that may work in a wide range of temperatures: -70° to 600°F (-56° to 315°C). The presence of graphite filler in Meldin^® 7500 material expands its operational envelop towards higher velocity regimes. Additionally, the self-lubrication nature of H310^® and Meldin^® 7500 products have shown to extend wear life when compared to conventional metal solutions. 

Omniseal Solutions’ high performance, composite materials are precise fit, custom solutions with high temperature stability and mechanical strength that can be used under dynamic conditions and environments. If the presence of fibers may result in high wear rate or excessive heat at high speeds, Omniseal Solutions does provide another versatile solution with the Meldin^® 7000 series polyimide material.